Fundamental measurement procedures for establishing optical tolerances for ground-based telescopes

Abstract

Fundamental measurement procedures are described that enable full end-to-end characterization of the entire imaging path between astronomical objects and their telescope images. The procedures are based on measurements of certain key properties of point-object (unresolved star) images, properties that carry the essential information about the integrated effects of all mechanisms in the imaging path that contribute to the OPD fluctuations. These mechanisms can include, but are not limited to, atmospheric turbulence, dome turbulence, telescope aberrations, the effect of wind-induced oscillations on large multiple-segment primary mirrors, and the (corrective) effects of Adaptive Optics (AO). The measurement procedures are fully general and take into account amplitude scintillation as well as phase variation in the wavefronts. The effects of atmospheric turbulence are fully encrypted in the statistical properties of the OPD fluctuations in the telescope pupil. In turn, these OPD properties are fully encrypted in the measured point-object image properties. Consequently, there is no need to make a priori assumptions about the form of the turbulence structure function, Kolmogorov or otherwise; the measurement procedures naturally provide the appropriate structure function. Whereas telescope aberrations usually contribute fixed amounts to the total OPD fluctuation, other mechanisms, such as atmospheric turbulence, contribute time-varying amounts. The measurement procedures allow the fixed OPD contributions to be separated from the time varying contributions. If AO is used, it is appropriate to have the AO system running during the measurement procedures, which otherwise remain unchanged. The procedures described are fundamental to establishing appropriate end-to-end wavefront error budgets for ground-based telescopes and to establishing telescope resolution expectations as a function of wavelength.